Rickettsia parkeri and Candidatus Rickettsia andeanae in Tick of the Amblyomma maculatum Group, Mexico

We report Rickettsia parkeri and Candidatus Rickettsia andeanae in ticks of the Amblyomma maculatum group collected from dogs in Sonora, Mexico. Molecular characterization of these bacteria was accomplished by DNA amplification and sequence analysis of portions of the rickettsial genes gltA, htrA, ompA, and ompB.

We report Rickettsia parkeri and Candidatus Rickettsia andeanae in ticks of the Amblyomma maculatum group collected from dogs in Sonora, Mexico. Molecular characterization of these bacteria was accomplished by DNA amplification and sequence analysis of portions of the rickettsial genes gltA, htrA, ompA, and ompB. R ickettsia parkeri, a member of the spotted fever group Rickettsia (SFGR), was initially identified in Amblyomma maculatum ticks in 1937, but not until 2004 was the first confirmed human infection reported (1). Through 2018, at least 9 cases of R. parkeri rickettsiosis have been identified across several mountainous locations in southern Arizona close to the US-Mexico border, and ticks of the A. maculatum group infected with R. parkeri have been collected from this region (2).
Epidemic levels of Rocky Mountain spotted fever (RMSF), a severe and frequently fatal spotted fever rickettsiosis, have been identified throughout regions of northern Mexico, including the state of Sonora (3). R. rickettsii (the agent of RMSF) is the only SFGR species linked with spotted fever rickettsiosis in Sonora; nonetheless, a unique lineage of R. parkeri associated with the Dermacentor parumapertus tick was reported recently from northern Mexico (4). Sonora shares an international border with Arizona; the recent identification of R. parkeri in Amblyomma ticks in southern Arizona (2) prompted our investigation during July 2016 for ticks in northern Sonora to determine whether another pathogenic SFGR species could also exist in Mexico.
We collected a total of 31 A. maculatum group ticks from northern Sonora We amplified a sequence demonstrating complete identity to the homologous 590-bp segment of the ompA gene of R. parkeri strain Maculatum 20 (GenBank accession no. U43802) from 1 (7%) of 14 ticks collected in 2016. We further amplified sequences demonstrating complete identities to the homologous segments of the ompA (590 bp) and 17-kDa antigen (208 bp) genes of Candidatus Rickettsia andeanae (GenBank accession nos. KF179352 and KY402193, respectively) from another tick in this group and sequences revealing complete identity to each other and to the homologous segments of the gltA (760 bp), ompB (760 bp), and ompA (490 bp) genes of R. parkeri strain Portsmouth (GenBank accession no. CP003341.1) from 10 (71%) of 14 ticks evaluated from the 2017 collection.
We identified DNA of R. parkeri and Candidatus Rickettsia andeanae in A. maculatum group ticks in northern Sonora. R. parkeri causes a disease less severe than RMSF and should be suspected in patients with an eschar, rash, and lymphadenopathy (1). The results of this investigation suggest that R. parkeri could contribute to at least some of the cases of spotted fever rickettsiosis described in Sonora and possibly in other regions of Mexico where A. maculatum group ticks are found. Differentiation between these 2 diseases is important, principally because there are no reports of fatal disease caused by R. parkeri. Nonetheless, clinical suspicion of any SFGR requires immediate treatment with doxycycline. Candidatus Rickettsia andeanae, a SFGR of undetermined pathogenicity, has been detected in the United States and Central and South America (2,10). Our findings highlight the need for specific diagnostic tests for SFGR in Mexico that can identify other potential SFGR of public health concern in this country.

About the Author
Dr. Delgado-de la Mora is a researcher and resident of Pathology at Instituto Nacional de Ciencias Médicas y Nutrición Salvador Zubirán. His research interests include tickborne diseases, particularly those caused by Rickettsia spp.